In another sense - radar used to first detect an object within, say, 1 million km - I suspect the difficulty is more to do with 'illuminating the sky' than anything else. The methods used to get radar images of Saturn's rings won't really work if your objective is to spot a large meteor closing fast on the Earth.

Reflections are interesting. Reflected signal is allways a signal from the history. Radar reflection from Titan tells (among other thigs) that 2.25 hours back in the history some people were experimenting with a radar.

Reflection of sunlight from Pluto tells something about the Sun approximately 10 hours ago.

Could the developments in detecting reflected radiation reach in the future days or even years?

Research of such reflections would provide us a new approach in search of the past?

Originally posted by Nereid No better than taking a picture now and developing the film tomorrow!

Picture taking works only when you know that this is the moment and topic you want to watch later. Same is valid for video.

Instead given that basically all celestial objects are reflective, among the incoming radiation there must be (very) weak signals that are reflections from the Sun or even our planet. And the reflections are there also in the case you didn't bring the camera with you.

From a single reflector at the distance of N light years, we will basically get continuous stream originated 2N years ago. Various distances enables us to "see" over various distances in time.

The question is, are there such reflective surfaces big enough.

If one can speak about gravitational lenses, why can't we assume the existence of gravitational reflectors...

"If one can speak about gravitational lenses, why can't we assume the existence of gravitational reflectors..."

Because, in most basic terms, of the four forces (Strong Nuclear, EM, Weak Nuclear and Gravity), only the first three are known to act as both a repulsive and attractive forces. Gravity is attractive only. So, if "gravity waves", Gravitons, etc. approach a body they are attractive-only and would interact as such and not "reflect" back off that body. They (it, gravity) might change a property of the other body, such as its motion, and we might be able to detect that change by other means. But, the only "gravity" we could detect from the other body would be the influence of its own gravity/mass.

IIRC, there was some discussion recently about black holes acting like very weak reflectors, so that if you looked at the black hole, you would see a (very faint) image of the Sun - light goes out to the black hole, gets bent 180o, and we see the bent light much later.

In practical terms, this isn't very helpful, at least to what bothnicum is looking for. For a start, the black hole 'reflection' ties you down just as much as photos (you can't vary the time delay - that's fixed by the distance to the black hole). Then there's the small matter of actually seeing the 'reflection' - the Sun, as seen from Earth is mag -26; a black hole reflection would likely be about as faint as the faintest galaxies in the recent Hubble Ultra Deep Field (~30 mag), > 20 orders of magnitude!